The present invention relates to heat-resisting furnace rolls used to transport the work such as steel sheets and the like through heating furnaces or heat-treating furnaces.
Rolls used to transport the work through heating furnaces or heat-treating furnaces such as reducing furnaces for performing chemical reduction prior to galvanizing, continuous annealing furnaces for electrical sheets, or annealing/pickling furnaces for stainless steel sheets are in most cases exposed to hot atmospheres in the range of 500.degree. to 1300.degree. C. In order to withstand such hot atmospheres, these conveying rolls are either made of heat-resisting materials such as Ni-Cr or Co-Cr base alloys, or consist of a support roll having a thermal sprayed coat of a ceramic material such as alumina or zirconia. One problem with heat-resisting steel rolls is that foreign materials from the work become fused to the roll surface and eat into the surface of the work to cause flaws in it. This problem is absent from rolls with a thermal sprayed ceramic coat. However, because of spalling of the ceramic coat, the service life of this type of roll is as short as 2 to 3 months. Furthermore, once spalling of the ceramic coat occurs, foreign materials can build up on the exposed surface of the support roll and cause the development of flaws in the work surface.
An improved heat-resisting roll that is said to have the ability to solve these problems has recently been proposed. This roll consists of a ceramic sleeve (sleeve made of a compacted and sintered ceramic powder) fixed to a support roll with keys or pins, and an integral combination of the support roll and ceramic sleeve ensures in-phase rotation wherein the peripheral speed of the roll is in synchronism with the work transport speed. Among the many ceramic materials proposed for use in the sleeve are alumina, silicon carbide, and silica.
The first proposal for such a ceramic sleeve was made more than ten years ago. However, because of its many defects, e.g. the insufficient mechanical strength of ceramic materials, cracking in or spalling of the sleeve due to the difference in thermal expansion coefficient from that of the metallic roll body, and vulnerability to thermal shock, a heat-resisting roll with this ceramic sleeve has yet to be fully commercialized. One reason is that a roll consisting of an integral combination of a support roll and a ceramic sleeve fitted over it experiences transverse deflection due to the difference in thermal expansion between the support roll and sleeve. Another reason is that the ceramic used is a brittle material and the sleeve soon becomes unserviceable due to brittle fracture. Furthermore, in a roll consisting of a support roll and a ceramic sleeve that is fitted over it and fixed thereto with keys or pins, impact resulting from the turning of the roll causes cracks to develop in the vicinity of a key hole or pin hole and such cracks propagate into the sleeve surface.
U.S. Pat. No. 3,751,195 discloses a heat-resisting roll consisting of a support roll surrounded by a porous sleeve of fused silica. However, the support roll and the sleeve are integrally coupled by keys in keyways so as to operate as a single unit, and cracking originates unavoidably in the vicinity of the key ways. Therefore, in spite of the many advantages of fused silica in the ceramic sleeve, the roll of that patent publication has not yet been commercialized.
A heat-resisting roll consisting of a support roll and a ceramic sleeve fitted over it and secured thereto is disclosed in Japanese patent publication No. 48086/1980. By using a sintered product of a "fine ceramic" powder comprising particles with an average size of 5-10 microns, this roll is capable of withstanding use in hot atmospheres in the range of 1500.degree.-1600.degree. C. The thermal expansion that will take place in such hot atmospheres is absorbed by two methos: (1) the ceramic sleeve is divided into segments in the axial direction of the roll and a clearance is provided at both ends of the roll; (2) a certain clearance is provided between the inner wall of the sleeve and the outer wall of the support roll along their entire length. In order to absorb the transverse deflection and the thermal expansion that will occur in the radial direction of the support roll, it is particularly important that a certain clearance be provided between the sleeve and support roll in their upper section. However, this arrangement necessitates a complex structure.